Over the decades, there has been a concerted effort to reduce atmospheric pollution caused by volatile solvents which are emitted during painting processes. Due to environmental concerns, volatile organic compounds (VOCs) have come under strict regulation by the government. Therefore, one of the major goals of the coating industry is to minimize the use of organic solvents by formulating waterborne coating compositions which provide a smooth, high gloss appearance, as well as good physical properties including resistance to acid rain. While the solvent-type coatings provide many benefits, such as that they are fast-drying, have a high hardness, a high abrasion-resistance, a high water-resistance, a high chemical-resistance and a low price, the waterborne coatings have environment-friendly benefits in that they are not flammable or explosive. The waterborne coatings use water as the system solvent and contain no poisonous chemicals. They require no or low amounts of volatile organic compounds.
In this context, aqueous polyurethane (PU) dispersions or emulsions have been replacing the solvent-type products in the fields of coatings, adhesives and ink. However, pure aqueous PU dispersions have some inherent drawbacks. In order to form a stable aqueous dispersion, the hydrophilic groups, for example, carboxylic, sulfonic, ammonium and other ionic groups should be introduced into the PU molecular chains. This modification however may lead to lower water-resistance of the aqueous PU dispersion compared to the solvent-type products. Therefore, it is necessary to modify the aqueous PU dispersions with other hard polymeric materials such as polyacrylates.
Polyacrylates (PAs) are the most important polymers used in modifying the PUs. The PA and the PU are complements in many aspects. The PU is advantageous in that it is cold-resistant, solvent-resistant, elastic and flexible, glossy and exhibits little change in hardness with changes in temperature, while the PU is disadvantageous in that it has low water-resistance and low mechanical strength. The PA is advantageous in that it has high mechanical properties and is anti-yellowing, aging-resistant and water-resistant, while the PA is disadvantageous in that it has low solvent-resistance and that the PA film is tacky under high temperature but brittle under low temperature. Accordingly, obviously the properties of the PU and the PA complement each other. The composite materials of PU and PA are more outstanding in terms of adhesion, film-formability, non-stickiness, weather-resistance, elongation and strength of the film than that of either the PA or the PU taken alone. Accordingly, since the development of PU, the modification of the PU by the PA has remained an active research topic in the art.
There are two kinds of modification to the PU by the PA: a physical method and a chemical method. The physical method is achieved by mechanical mixing. In the physical method, aqueous PA and PU dispersions (emulsions) are independently prepared first, and then both dispersions are mixed together under mechanical power. A high speed mechanical stirrer may be used for this purpose. It is a very convenient method that makes it easy to control the particle size and the stability of the dispersions. However, the properties of the mixed dispersion are often not very satisfactory, because the particles of PU and PA are separated in the dispersion and the phase separation occurs during film-formation. (D. Kukanja et al, J. Appl. Polym. Sci. 78, 67, 2000)
For these reasons, the chemical modification technology currently plays a more important role. The chemical method is achieved by post-polymerization of acrylates. In the chemical method, the PU dispersion can be prepared first, and then acrylates and other vinyl monomers can be polymerized in the PU dispersion. In most cases, core-shell emulsion polymerization is adopted. PU particles are used as core particles and the acrylates are polymerized in the PU particles due to high hydrophobicity of the acrylates. Therefore, the inverse core/shell structure is always obtained.
In addition, the free radicals are typically easily captured by N—H groups in the PU chains. As such, the PU can also serve as a chain transferring agent in the acrylates polymerization, and a graft of PA chains to PU chains is formed. Furthermore, if multi-double-bond (>2) unsaturated monomers are used in the emulsion polymerization, or multifunctional polyols or polyisocyanates are used in the preparation of PU, crosslinking and interpenetrating networks are formed in the core-shell particles. This leads to enhanced compatibility between the PU and the PA and the phase separation is minimized or even eliminated. This is likely the reason why the properties of the PU-PA composites produced by the chemical method are much better than those of the mixtures of both materials obtained by the physical method.
In the preparation of aqueous PU dispersions by the chemical method, a high viscosity for the prepolymer melt should be avoided so that good operability can be achieved in obtaining PU dispersions with a high molecular weight. For this purpose, such volatile organic solvents as acetone, methyl ethyl ketone, etc. are typically used in the reported literature or disclosed methods in the art. With the use of these solvents, the viscosity can be maintained at a low level. At the end of the preparation of aqueous PU dispersions, the solvents are typically removed from the dispersions under vacuum pressure.
For example, U.S. Pat. No. 5,556,912 discloses a preparation of aqueous binder dispersions including polyurethanes and water-based physically drying coating compositions containing these aqueous dispersions which are particularly suitable for plain and metallic colored base coats employed for automotive repair lacquer coatings. In the patent, the reaction proceeds in the presence of a solvent or in the presence of so-called reactive diluents, such as methyl ethyl ketone, methyl isobutyl ketone, acetone, tetrahydrofuran, toluene and xylene, which are subsequently eliminated by distillation.
In the chemical method, certainly acrylates and other ethylene unsaturated monomers without active-hydrogen in the molecules can be used as a solvent at first, and then they can be radical polymerized after the prepolymer of PU has been dispersed in water. For example, U.S. Pat. No. 7,001,952 discloses a preparation of aqueous PU dispersions including PU-acrylate particles dispersed in an aqueous medium.
In the stage of preparing the PU prepolymer by the chemical method, for example, a mixture of methyl methacrylate and butyl acrylate can be added into a reactor, so the viscosity of the reaction system is very low. After water is fed into the reactor and the PU prepolymer is extended and dispersed, radical initiators are added into the dispersions to create the emulsion copolymerization of methyl methacrylate and butyl acrylate. In said U.S. Pat. No. 7,001,952, in order to enhance the dispersibility and to reduce the particle size of the dispersion, a microfluidizer is used after the dispersion. The initiators adopted in the patent are of the redox-type, i.e. t-butylhydroperoxide/ferrous ammonium sulfate/sodium metabissulfate and isoascorbic acid/hydrogen peroxide.
In another U.S. Pat. No. 6,635,706, polyols, dimethylol propanic acid, bis(4-isocyanatocyclohexyl)methane and N-methylpyrrolidinone (cosolvent) are used in the preparation of PU prepolymer. A mixture of methyl methacrylate and butyl acrylate is added into the solution and then dispersed in water. At the final stage, emulsion polymerization is carried out under 75° C., by using Vazo-52 [2,2′-azobis(2,4-dimethylpentanenitrile)] as an initiator, and then using ammonium persulfate, tertiary butyl hydroperoxide, cumene hydroperoxide and hydrosulfite successively to accelerate the polymerization.
In addition, Japanese Patent No. 9 165 407 and U.S. Pat. No. 6,635,706 disclose that active acrylates, i.e., acrylates with hydroxyl groups, can react with a —NCO terminated polyurethane prepolymer so that vinyl groups can be introduced into the prepolymer chains. The advantages of this technology are: (i) the prepolymer with double bonds can copolymerize with acrylates in the later stage so that the PU and the PA can be crosslinked; (ii) NCO groups in the prepolymer are consumed by the hydroxyl groups in the active acrylates; and (iii) extending reaction of the prepolymer in the water can be ignored so that the dispersed phase in the aqueous PU emulsion is of low molecular weight materials, even oligomers of PUs. Thus, a stable and uniform emulsion can be obtained easily.
The PU can be modified by the PA and other polymer resins together. In U.S. Pat. No. 5,688,859, PAs and alkyd resins with hydroxyl and carboxylic groups are synthesized first, and then diisocyanates react with a mixture of the two resins in solution. Finally, neutralization and dispersion are conducted subsequently. The PU/PA hybrid emulsions obtained are in good appearance and the polymer film is of excellent properties. The solvent used in the manufacturing, however, is more than 5% in the product.
U.S. Pat. No. 6,166,150 also discloses that alkyd resins and PAs are used to modify the PU. The alkyd resins and the PU were prepared in organic medium, respectively, and then were blended. The solution was neutralized and dispersed in water afterwards. Acrylate monomers were then copolymerized in the alkyd resin-PU dispersion. The radical polymerization of acrylates would last for 10 hours.
Many more patents and research papers disclose the preparation of aqueous PA modified PU dispersions. However, the balance of polymer properties and operation complexity is still a challenge. Although the emulsion polymerization of acrylate monomers always leads to excellent properties, either the use of organic solvents or the tedious operations have to be considered as elements requiring improvement. The productivity is therefore markedly limited. In the PU industry, in fact, antioxidants are necessary to be added into most polyols, which will inhibit the radical polymerization of vinyl monomers drastically. Furthermore, as mentioned above, although the N—H groups in the PU chains are in favor of the grafting reaction of PA to PU, this “side reaction” will slow down the radical polymerization because the radicals are very easily transferred to PU chains, leading to the low conversion of the acrylate monomers into polymerization. As a result, the monomer conversion of the acrylates emulsion polymerization in the presence of PU is very low, ca. 60˜70%. In order to enhance the monomer conversion, some intensified strategies should be adopted in the radical polymerization stage, for example, either to extend the reaction time, or to use composite initiators as in U.S. Pat. No. 6,635,706. Besides, some PA/PU hybrid emulsions are yellowish or light brown. The appearance of these products looks unattractive. Therefore, it is an urgent need to conveniently produce PA/PU hybrid dispersions to obtain a high yield of the desired product.
In Chinese Patent No. 1 597 739A, a method is disclosed to prepare the PA emulsion in advance and then add the PA emulsion into a PU prepolymer solution directly during the dispersion stage. The method is easy to control and the obtained PA/PU hybrid bears enhanced properties. However, according to this method, volatile organic solvents must be used in the PU prepolymerization stage and the volatile organic solvents must be removed under vacuum at the end of process.
Meanwhile, as a way to achieve the desired properties of the PA/PU composite materials, chemical crosslinking between the PA and the PU chains has been employed. In this regard, one effective and feasible strategy is to adopt a two-packaged PA/PU system. There are quite a few patents found for two-packaged aqueous PA/PU coatings production, such as U.S. Pat. Nos. 6,515,068, 5,670,600 and 5,876,802.
U.S. Pat. No. 5,675,600 discloses a two-packaged PA/PU system. In this system, water-dilutable acrylates copolymers are prepared via solution polymerization using ethoxyethyl propionate; the polyacrylates are dispersed in water; and then the organic solvent is finally removed from the dispersion by azeotropic distillation. The aqueous polyacrylate dispersion can be blended with a water-dilutable polyisocyanate commercially available so that a transparent topcoat can be obtained. It is obvious that in this two-part PA/PU coating system, the PU component is used as a crosslinker and the PA is the main resin for the coating. Therefore, the properties of the film will mainly depend on the polyacrylate component.
U.S. Pat. No. 5,876,802 is an updated version of U.S. Pat. No. 5,675,600, with the essential strategy not changed.
U.S. Pat. No. 6,515,068 also uses the water-dilutable polyisocyanate as a crosslinker for the polyacrylate with hydroxyl groups.
Although two-packaged coatings provide a high performance, such coatings are not convenient for painting operations. Two components must be stored in separate containers and they must be mixed in accordance with a specific ratio before being applied. In this regard, the pot life of the “finished” coating products is always shorter than 3 hours. Therefore, a one-packaged product will tend to be superior to a two-packaged product in terms of operability.
Further, it will be much better if the crosslinking of PA and PU chains can be achieved under ambient temperature. For self-crosslinking systems under ambient temperature, some pairs of groups can be considered, for instance, epoxide/primary amine and acetoacetoxy systems.
U.S. Pat. No. 6,063,861 discloses a preparation of a pure polyurethane aqueous dispersion using a hydroxyl carboxylic acid method. In this patent, the PU is extended with amines and then the acrylates monomers and Acetoacetoxyethyl methacrylate (AAEM) are polymerized in the presence of the PU dispersion. The results of the patent show that AAEM in this system can lead to the self-crosslinking of the PA/PU hybrid dispersion.
In the present application, a method of preparing an aqueous PA modified PU dispersion with high stability is provided, without the use of volatile organic solvents but with high productivity and convenience in operation. Further, as a way to improve the properties of the modified PU dispersion, a method of preparing an aqueous PA modified PU dispersion with self-crosslinkability is provided.